Coal is the most abundant natural energy source in the world. A significant portion of the U.S. domestic energy requirements are met by burning coal as a fossil fuel. There are various types of coal found within the U.S., i.e., anthracite, semi-anthracite, low-volatile bituminous coal, medium- and high-volatile bituminous coal, sub-bituminous coal, and lignite. Coals such as anthracite and semi-anthracite typically have high ash and sulfur contents and therefore require beneficiation prior to use.
The primary purpose of coal beneficiation is to reduce the incombustible ash content, thus enhancing the heat content. Reduction in the ash content results in savings in transportation and ash disposal costs. Sulfur, mainly in the form of purite, is also reduced.
Another important economic factor to be considered in coal processing is the recovery and reuse of process water. Water is typically very expensive and there are often limits on total usage. Also, strict environmental controls prohibit or severely limit discharge of process water. Thus, it is imperative that solids be efficiently removed from the process water and water recycled to the process stream.
Beneficiation of coal is effected using two primary properties of coal, i.e., (1) differences in specific gravity between coal and its impurities, and (2) differences in surface characteristics between coal and its impurities. Since the higher ash content fractions are usually found in the finer coal sizes, some plants only screen out these sizes to beneficiate the coal. However, since the quantity of such fine coal is on the rise, even this is treated.
A coal beneficiation plant may be broadly divided into specific gravity separation and fine coal treatment. In gravity separation, cleaning units make use of the differences in specific gravity between coal and its impurities to effect separation. Normally, the specific gravity of the clean coal is less than that of its impurities. Some examples of commonly used equipment for gravity separation are: jigs, heavy medium baths and cyclones, washing tables, water-only cyclones and spirals.
Fine coal treatment incorporates a flotation cell(s), clean coal filter and thickener. In the flotation cell, a collector and frother are added to the flotation feed. A collector such as diesel oil selectively imparts hydrophobicity to the coal particles. This increased hydrophobicity makes the air bubbles more likely to attach to the coal particles. The frother, generally an alcohol-based product, reduces the surface tension of the air/water interface, thus making a stable froth.
The clean coal concentrate from the flotation cells goes to the clean coal filter and is dewatered. The tailings from the flotation cell go to the thickener where they are thickened and discharged.
The thickener is treated with coagulants and flocculants to enhance settling. Typically, the coagulants and flocculants are added at several points along the feed line to the thickener and in different sequences. Coagulation is the destabilization by surface charge neutralization of stable negatively charged particles that are in suspension (i.e., settleable or dispersed) through the utilization of inorganic salts or cationic polyelectrolytes. Flocculation is the aggregation of finely divided particles which are suspended in a liquid through the utilization of an entrapping agent, generally an inorganic flocculant, or a bonding agent, generally an organic flocculant, that brings the particles together.
During the processing of coal, a coal refuse slurry is generated. This slurry consists of residual coal fines and clays suspended in plant process water. Due to the high volume of water used in the processing of coal, it is necessary to reclaim the wash water for recirculation in the plant. The concentrated solids are sent to an impoundment pond for disposal. Generally, the use of anionic flocculants is sufficient to remove the majority of the coal fines; however, when there are high levels of clay in the mined coal, it is necessary to supplement the use of anionic flocculants with the use of cationic coagulants. The sequential addition of flocculants and coagulants is used primarily in the coal refuse thickener and in the subsequent twin belt press filtration of the thickener underflow. The typical application order in the thickener, which is similar to a clarifier, is coagulant addition followed by flocculant addition. This provides a controlled turbidity of the recycle process water and a controlled solids settling rate. In the thickener underflow filtration, the order of application is normally flocculant followed by coagulant. This treatment gives highly agglomerated solids which provides effective dewatering of the waste solids and low turbidity in the recycled process water.
The decrease in sludge volume or the increase in sludge solids results in more efficient use of plant process water and a reduced loading in the impoundment pond. The impoundment pond is the area of the mine where the sludge is used to landfill existing mined surfaces. With time, the sludge further compresses in the impoundment area which provides reclaimed mine sites.
The typical equipment used for sludge thickening and dewatering in the coal industry are Gravity Thickeners, Twin Belt Presses, and Rotary Drum Filters. Each of these pieces of equipment uses flocculants and coagulants. The doses of flocculant and cationic polymers are 5-10 ppm and 1-5 ppm, respectively, for the thickener and 5-20 ppm and 5-30 ppm, respectively, for the filter applications. These polymers are highly surface active and they remain with the solids that are sent to the impoundment pond. These products are used in closed loop coal refuse treatment applications. A treating polymer is also required for the dewatering of other mining underflow solids such as copper ore refuse slurries.
In addition to the treatment of fine coals, dewatering is also necessary in other areas of mineral processing. A variety of mineral slurries such as taconite, trona, sand and gravel slurries and titania require solids removal and dewatering. The same basic processing steps are utilized to extract titanium oxide from titania, for example.
Although some inorganics, principally alum and iron salts, are still used as coagulants, water soluble organic polymers are now more prevalent. Both naturally occurring and synthetic polymers find use as coagulants and flocculants in the mining industry. The principal natural polymers used are starch and guar, both of which are high-molecular weight polymers of simple sugars, such as polysaccharides. Starch is a polymer of glucose consisting of a mixture of linear (amylose) and branched segments (amylopectin).
Synthetic polymers are advantageous because they can be tailored to a specific application. This has resulted in a wide range of commercially available coagulants and flocculants of varying charge, composition, and molecular weight. The most widely used synthetic coagulants are polydiallyldimethylammonium chloride (polyDADMAC) having molecular weights in the range of from 100,000 to as high as 1,000,000 or higher and condensation polymers of dimethylamine and epichlorohydrin (Epi/DMA) which generally have molecular weights in the range of 20,000 to 100,000.
The most widely used synthetic coagulants are polydiallyldimethyl ammonium chloride as described in U.S. Pat. No. 2,926,161 and condensation polymers of dimethylamine and epichlorohydrin such as those described in Reissue U.S. Pat. Nos. 28,807 and 28,808, though co-polymers of 3-acrylamido-3-methylbutanoic acid and acrylamide or acrylic acid are also known, as described in U.S. Pat. No. 5,296,006. These polymers vary greatly in molecular weight, typically ranging from several thousand to as high as 500,000. Condensation polymers are made in solution form, and are available commercially as aqueous solutions containing a relatively low weight percent polymer. Polydiallyldimethyl ammonium chloride is a vinyl addition polymer, which (at the molecular weights used for coagulation) has also been made in solution form. Typical commercially available polydiallyldimethyl ammonium chloride is available in aqueous solutions containing 1-20% by weight polymer.
Copolymers of diallyldimethylammonium chloride and acrylamide having utility for the dewatering of mineral slurries have been described in U.S. Pat. No. 4,673,511. Effective co-polymers consisting of co-polymerizing diallyldimethylammonium chloride (DADMAC) with various other monomers are also known. For example, U.S. Pat. No. 5,330,546 discloses effective co-polymers of DADMAC with a dialkylaminoethylacrylate or a dialkylaminoethylmethacrylate; U.S. Pat. No. 5,476,522 discloses effective copolymers of DADMAC with a vinylalkoxysilane; U.S. Pat. No. 5,653,886 discloses terpolymers formed from polymerization of monomers including DADMAC and U.S. Pat. No. 5,518,634 discloses polymers formed from DADMAC and acrylamide, cross-linked with triallylamine.
Copolymers of DMAEA.MCQ (dimethylaminoethylacrylate methyl chloride quaternary salt) and acrylamide have been described as flocculants in U.S. Pat. No. 4,720,346. Copolymers of DMAE(M).MCQ and DADMAC as treatment materials are described in U.S. Pat. No. 4,835,206.
Dry water soluble polymers such as dry polydiallyldimethyl ammonium chloride have also been used to dewater coal refuse slurries. These polymers have met with some success, dissolving in the refuse thickener over a period of 45 to 60 seconds. Such time is long enough to provide continuous feed of fresh polymer into the coal/clay slurry.
Homopolymers in solution or solid form produced by polymerizing dialkylaminoalkyl(meth) acrylates quaternized by methyl chloride improving the stability of coal dispersions and assisting the separation of coal from mineral matter are described in U.S. Pat. No.4,370,429. Moreover, dry homopolymers formed from dialkylaminoalkyl(meth)acrylamides as quaternary ammonium salts have been disclosed in U.S. Pat. No. 5,178,774. That reference also teaches that some starch may be utilized as a carrier to promote formation of particles with the cationic polymer. Yet that reference teaches that for best efficiency, starch should not be used, and therefore contradicts the findings described herein.
Starch is described as a dehydrating agent for a coal froth in Kokai No. 59-122594. However, combinations of starch and polymer are not indicated in this reference, and as will be demonstrated by the disclosed examples, the combination of polymer with starch results in an unexpectedly large increase in performance.
Therefore, since there is still a need to achieve better performance, leading to better clarity, reduced consumption of polymer, or simpler operation, the use of polymer/starch combinations as described herein is desirable as they result in improved efficiency of processes in the mining industry.